This invention relates to a rotary impact power tool that delivers in rapid succession a series of rotary impact forces or blows. Tools of this type are typically used to tighten or loosen high torque nuts or bolts or similar items.
A conventional rotary impact wrench mechanism, known as a "swinging weight" mechanism, is disclosed in U.S. Pat. No. 2,285,638, issued to L. A. Amtsberg. While this mechanism was rather inefficient, it was one of the first to deliver rotary force in a series of impact blows. The ability to deliver a series of impact blows offers a human operator a tremendous advantage in that the operator can physically hold the impact wrench while delivering very high torque forces in very short bursts or impacts. The advantage of applying short duration high torque impact blows is that a normal human being can continue to physically hold the tool while applying very high torque forces. If the torque were applied continuously, it would result in an opposite continuous reaction force on the tool that would be far too great to be held by a normal human being.
The "swinging weight" mechanism was greatly improved upon by the invention of Spencer B. Maurer as disclosed in U.S. Pat. No. 3,661,217, which is hereby incorporated by reference. This patent describes a swinging weight impact wrench mechanism with a hammer member that is substantially free of tensional stresses during impact. The Maurer "swinging weight" mechanism has a swinging hammer pivoted on a novel type pivot with a center of mass of the hammer near the center of rotation of the mechanism. This enables the swinging weight mechanism to strike a more balanced blow to an anvil and, ultimately, to the output shaft to tighten or loosen bolts, for example.
The problem with the Maurer mechanism is that the curved impact surfaces between the hammer and anvil on the inside of the tool where the bursts of torque are generated, are forced to absorb high forces and stresses. This causes durability problems, loss of transmission of energy into the joint, and improper operation of the mechanism.